A gas fuel tank is for filling combustible gas, and generally provides three gas outlet modes of different states according to requirements. These modes include an outlet off mode, a metering outlet mode and a continual outlet (gas filling) mode. In the metering outlet mode, when a corresponding structure is pressed to a predetermined depth, a quantitative amount of gas is supplied to satisfy the use of a predetermined purpose. In the continual outlet mode, a gas is continually supplied for usage or as a filling gas. In the metering outlet mode, a user is not required to control an output dosage of a gas, such that not only the amount of gas used can be saved but also application safety can be enhanced.
The metering outlet mode, featuring effects of saving resources and being safe and reliable, is one mainstream in using gas fuel tanks. For example, the U.S. Pat. No. 5,169,038, “Metering Valve Usable in the Upsidedown Position”, discloses a metering gas outlet structure. In a structure of the above disclosure, two gas channels are respectively disposed above and below a gas guiding member. With a metering chamber in the middle, the gas channels above and below respectively become in communication with the metering chamber to quantitatively discharge a gas. However, such type of metering structure is directly formed on a gas can during a manufacturing process in a way that a user cannot select a metering outlet mode or a continual outlet mode as desired, and may thus provide insufficient convenience.
Further, for example, the U.S. Pat. No. 6,302,297 B1, “External Metering Valve for a Fuel Cell”, discloses an external metering valve. The above disclosure may be externally connected to a gas fuel cell, and quantitatively supplies a gas for usage by using a horizontal external metering valve. A gas in the gas fuel cell enters the metering valve through a horizontal transfer pipe, which may cause an excessively long path in between. Further, a gas is blocked and prevented from entering by a sealing effect between an outer wall surface of a pipe for outputting the gas and the metering valve to achieve a demand of quantitatively outputting the gas. However, wear may likely occur due to multiple abrasions between the outer wall surface of the pipe and the metering valve, such that the sealing effect may be not be accomplished to fail the demand for airtightness.